Dry foods such as instant coffee, milk powder, tea, etc. are wrapped by an aluminum-deposited film having good sealability, etc., to be protected from oxygen and moisture. For example, an aluminum-deposited film 300 comprises, as shown in FIG. 16, a high-strength polyethylene terephthalate (PET) film 301, a print layer 302, a vapor-deposited aluminum layer 303, and a heat-sealing layer 304. As shown in FIG. 17, a bag 310 of an aluminum-deposited film is often provided with a notch 311 as a starting point of tearing.
However, an aluminum-deposited film cannot be torn easily in many cases, even with a notch 311, because of a vapor-deposited aluminum layer, a heat-sealing layer and a print layer formed on a PET film. Particularly because a heat-sealed portion 312 is twice as thick as other portions of the aluminum-deposited film, tearing starting from the notch positioned outside the heat-sealed portion 312 is often stopped by the heat-sealed portion 312.
In view of such circumstances, as a film easily tearable from any position without a notch, the inventor previously proposed by JP 7-165256 A an easy-to-tear plastic film comprising a porous film of polyester, nylon or oriented polypropylene, whose entire surface is provided with penetrating or unpenetrating pores having an average opening diameter of 0.5-100 μm at a density of 1,000/cm2 or more; and a heat-fusible polymer film laminated on a surface of the porous film. However, because this easy-to-tear plastic film has penetrating pores, it cannot be used for applications required to completely prevent the permeation of oxygen and moisture.
As a plastic film provided with unpenetrating fine pores, JP 10-193454 A discloses a tubular film of a polyolefinic resin composition containing an inorganic filler, which is as thick as 5-150 one or both of inner and outer surfaces of the tubular film being subjected to a corona discharge treatment, and at least part of the tubular film being embossed. Embosses (measured according to JIS B0601) are as deep as ½- 1/10 of the film thickness, and usually as large as 0.5-300 mm. However, because these embosses are too large, the tubular film has poor appearance. If small embosses were to be formed, an extremely expensive embossing roll having large numbers of small projections would have to be used, inevitably resulting in expensive embossed films.
When large numbers of fine pores are formed in a plastic film by a pattern roll and an anvil roll, as shown in FIG. 18, both of a pattern roll 10 and an anvil roll 20 were bent by a large load, resulting in a gap G wider in its transverse center portion. Fine pores formed by an uneven gap G have opening diameters and depths, which differ between the transverse center portion and peripheral portions, failing to provide a uniform microporous plastic film.
Accordingly, attempts have been made to reduce the bending of the pattern roll 10 and the anvil roll 20 by arranging backup rolls above the pattern roll 10 and/or below the anvil roll 20. However, because large numbers of high-hardness, fine particles are fixed to a rolling surface of the pattern roll 10, a rubber roll having a soft surface, etc. should be used as a backup roll, failing to sufficiently prevent the bending of the pattern roll 10 and the anvil roll 20.
JP 6-328483 A discloses an apparatus for calendaring a film of a thermoplastic polymer or rubber by six rolls arranged in substantially parallel, a sixth roll being arranged just under first to fifth rolls; and comprising a crossing means for inclining the fifth roll by a small angle. However, the angle of the fifth roll inclined by the crossing means is constant, and the calendaring apparatus does not have a driving means for obtaining a desired inclination angle. Accordingly, even if this calendaring apparatus were used to form unpenetrating fine pores in a plastic film, unpenetrating fine pores having various sizes and distributions cannot be formed uniformly in a width direction.